The present invention relates to soldering of elements such as printed wiring boards, and more specifically, the present invention relates to fluxless soldering in an nonexplosive gas atmosphere which substantially excludes oxygen.
When elements such as printed wiring boards and the like are soldered, it is invariably necessary to first apply a flux to promote wetting and good solder joints through removal of surface oxides on the metallic surfaces to be soldered. More recently inert gas soldering, generally nitrogen gas, sometimes with other inert gases have shown some benefits in the reduction of the amount of flux required in soldering. The inert gases cover the liquid solder surfaces and prevent oxidation of the solder. By eliminating flux and preventing oxidation of the solder, one avoids the necessity for cleaning the solder joints after soldering.
It has been suggested that a forming gas mixture which includes nitrogen and hydrogen has certain advantages for soldering, in that it is purported to promote hole filling in circuit boards without the use of flux. However, hydrogen in concentrations of 10% by volume or higher, presents handling difficulties and requires specific safety measures under certain conditions.
In U.S. Pat. No. 4,538,757 to Bertiger, soldering is accomplished in an atmosphere substantially free of oxygen and in a fluxing action occurs for soldering due to the presence of a gaseous reducing agent. A vent is shown from the center of an enclosure, and the gaseous reducing agent, referred to as hydrogen in one embodiment, is vented off through a burn off device. This seems to indicate that there is a sufficiently high proportion of hydrogen present that an explosive mixture may occur in the enclosure.
In our co-pending patent application filed Nov. 22, 1989, Ser. No. 441,009 is disclosed soldering in a shield gas atmosphere. The disclosure of this application is incorporated herein by reference.